The combustion of 1 kilogram of hydrocarbon requires approximately 3 kg of oxygen that means approximately 15 kg of air. Whatever is the technique, the key point of the burning is the mixing of the hydrocarbon with the huge quantity of air needed for the combustion.
In the case of gas, the mixture with air is rather easy to realize because the densities of both products are not very different.
In the case of liquid hydrocarbons, which have densities in the 700 to 950 kg/m3 range, the 15-to-1 air-to-hydrocarbon mass ratio means a minimum 10 000-to-1 volume ratio. With such a ratio, obtaining a mixture allowing a good combustion is difficult. All the developments of oil burner, made for several decades, had for objective the resolution of this problem.
In order to obtain a homogeneous mixture of liquid and air with such a volume ratio, it is necessary to split up the liquid in droplets (atomization) and to distribute these droplets homogeneously in the volume of air.
Moreover, the flowrate of liquid hydrocarbon to burn is at least 10 000 barrels a day or approximately 20 liters per second. The volume of air necessary for the combustion is then at least 200 m3 per second. The thermal power developed by the combustion of such an oil flowrate is roughly 600 megawatts.
A closed burner capable of burning such a flowrate would have a size, a weight and a cost unacceptable for an offshore installation, especially to be operated only during a few hours or days. Consequently, all the known burners work with free flames and are installed at the end of long booms to take the flames away from the platform, in order both to reduce the fire hazard and to decrease the thermal radiation of the flames towards the platform.
Existing burners use the pneumatic atomization which consists in breaking the liquid in droplets by injection of a strong compressed air flow in the liquid stream. The air-droplets mixture is then ejected in the atmosphere through an outlet.
With this technique, it is possible, through a constant size outlet, to change the air flowrate, by changing its pressure, and then to change the oil flowrate: by experience, it is possible to vary the oil flowrate in a 1 to 5 range.
This technique generates a strong air jet which, by friction in the atmosphere, absorbs a large quantity of atmospheric air, necessary for the combustion (phenomenon of ingestion).
Nevertheless, the quantity of oil which can be burnt in one single jet of compressed air remains low (by experience, around 2 liters per second) because physics limit the shape of the air jet to a narrow (around 15° angle) and short (around 7 meters) cone. Its contact area with the atmosphere remains small.
In such a conical air jet, if the flowrate of liquid is higher than 2 liters per second, the beginning of the flame is too rich with regard to the quantity of available air; the combustion is very incomplete, producing a lot of carbon and heavy unburnt products. The experience shows that a part of these products will burn later in the following part of the flame which continues to absorb atmospheric air, but another part will never burn, generating a thick black smoke and fall-out of unburnt hydrocarbons.
A technique used for several decades against this smoke (U.S. Pat. No. 3,894,831) consists in injecting a large quantity of water in the beginning of the flame, what gets rid of the smoke. Indeed, by cooling the beginning of the flame, the water slows down the phenomenon of evaporation of the droplets of oil, decreasing the apparent richness in the beginning of the flame. In a sense, the water allows a part of the droplets to pass thru the beginning of the flame and to go farther to burn. Unfortunately, a part of this liquid will never evaporate, or too late to burn, and will fall on the ground or at the sea, immediately or by later condensation of unburnt vapours contained in the flue gases.
A more recent solution (patent FR2741424, U.S. Pat. No. 6,027,332) consists in increasing strongly the air-to-liquid mass ratio in the burner, up to 18%. This reduces the richness of the jet and thus of the flame, with same air ingestion. The injection of water becomes unnecessary and the combustion is better, but the quantity of hydrocarbon burnt by one jet comes back to the 2 liters per second limit, what makes necessary a large number of jets: some ten flames, thus so many jets are necessary for the wanted flowrate.
That patent (FR2741424, . . . ) arranges twelve jets distributed following the shape of a wide cone around a unique point of distribution of the fluids.
Another patent (U.S. Pat. No. 5,993,196) arranges three groups of three jets from a structure of distribution of the fluids, the jets being arranged to distribute the nine flames in the largest possible volume.
Besides the cost of the compressed air (cost of compressors, occupied room, piping), the number of jets makes the burner complex and its maintenance expensive.
Moreover its range of flowrate remains narrow (1 to 5) except if it is possible to open selectively the outlets, what still increases the complexity of the system.
At last, the mixture of hydrocarbons and air inside the burner remains a safety issue.